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Thermodynamics

Short Answer Type

201. At 298K, enthalpy of fusion of sodium metal is +2.6 kJ mol^–1 and enthalpy of vaporisation of liquid sodium is + 98.0 kJ mol^–1. Calculate the enthalpy of sublimation of solid sodium at 298K.

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202.

A swimmer coming out from a pool is covered with a film of water weighing about 80 g. How much heat must be supplied to evaporate this water?

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203.

When 1 gm of liquid naphthalene (C₁₀H₈) solidifies, 49 joules of heat is evolved. Calculate the enthalpy of fusion of naphthalene.

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Long Answer Type

204.

How many grams of methane and volume of oxygen at N.T.P. would be required to produce 445.15 kJ of heat in the following reaction?

$CH subscript 4 left parenthesis straight g right parenthesis space plus space 2 straight O subscript 2 left parenthesis straight g right parenthesis space space rightwards arrow space space space space CO subscript 2 left parenthesis straight g right parenthesis space plus space 2 straight H subscript 2 straight O left parenthesis straight l right parenthesis semicolon$
$increment straight H space equals space minus 890.3 space k J$

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205. Discuss a method for the measurement of heat of combustion at constant volume (q_v or ∆E).

The heat of combustion at constant volume i.e. internal energy change is measured experimentally by Bomb Calorimeter. It is made up of heavy steel (Bomb) which can stand high pressure. It is surrounded by a bigger vessel which contains water and is insulated. A thermometer and a stirrer are suspended in it. The initial temperature of the water is noted. A known weight of the compound is taken in the platinum cup. Oxygen under high pressure is introduced into the bomb. A current is passed through the filament immersed in the compound. Combustion of the compound takes place. The heat energy evolved during the chemical reaction raises the temperature of water which is carefully recorded from the thermometer. By knowing the rise in temperature and heat capacity of calorimeter, the heat of combustion at constant volume can be calculated by using the expression,

where Q = Heat capacity of the calorimeter

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206.

Standard vaporisation enthalpy of benzene at its boiling point is 30.8 kJ mol^-1 ; for how long would a 100W electric heater has to operate in order to vaporise a 100g sample of benzene at its boiling temperature?

$open square brackets Power space equals space energy over time. space 1 straight W space equals space 1 space Js to the power of negative 1 end exponent close square brackets$

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207.

State and explain Hess's law of constant heat summation.

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208. How is Hess’s law helpful in calculating the enthalpy of formation of carbon monoxide?

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Short Answer Type

209. What is transition? How is Hess’s law helpful in determining the enthalpy of transition?

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210.

The thermochemical equations for the formation of water are:

$2 straight H subscript 2 left parenthesis straight g right parenthesis space plus space straight O subscript 2 left parenthesis straight g right parenthesis space rightwards arrow space space 2 straight H subscript 2 straight O left parenthesis straight g right parenthesis$
$increment subscript straight r straight H to the power of 0 space equals space minus 483. space 8 space kJ$
$2 straight H subscript 2 left parenthesis straight g right parenthesis space plus space straight O subscript 2 left parenthesis straight g right parenthesis space rightwards arrow space space 2 straight H subscript 2 straight O left parenthesis straight l right parenthesis semicolon space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space increment subscript straight r straight H to the power of 0 space equals space minus 571.8 space kJ$
How much energy is needed to convert 2 mol of liquid water to water vapour?